When a mini-cab driver ploughed into 11 pedestrians on a footpath near the Natural History Museum last month, there was an automatic assumption that this was another deliberate act of terrorism. With good reason. Three such car-ramming incidents have occurred in London already this year and we have been warned that further terrorist attacks are highly likely. Those who assumed it was a terrorist attack were wrong. More surprising was the police announcement that they were investigating a mere traffic accident.

Two other assumptions have proved to be wrong: firstly, that after the death of Osama bin Laden in Pakistan and the defeat of al-Qaeda and the Taliban in Afghanistan, peace and tranquility would return to the world; secondly, after the physical manifestation of the Islamic State caliphate was routed in Iraq and Syria, another swamp of terrorists would be drained. Wrong again. IS no longer encourages potential activists to join its ranks in the Middle East. Instead, it is advising them to wreak havoc in their home countries.

The truth, according to Lord Evans of Weardale, director-general of MI5 from 2007 to 2013, is that Britain is fighting a generational struggle against terrorism that will continue for another 20 to 30 years.

Gilles de Kerchove, the counter-terrorism chief of Europol, the European Union’s law-enforcement agency, estimates that there are now more than 50,000 jihadists in Europe. Britain leads the way, with an estimated 25,000. Of these, 3,000 are said to be of particular concern to the overstretched MI5, with just 500 receiving “constant and special attention”. By comparison, France is estimated to be home to 17,000 Islamic radicals, while Spain has about 5,000 and Belgium around 2,000.

De Kerchove noted that many plots had been foiled, “but, of course, some have succeeded . . . There is very sadly nothing like 100 per cent security. You can never prevent someone picking up a knife in the kitchen, leaving his apartment, and killing someone in the street.”

In a recent address, Britain’s most senior counter-terrorism police officer, Assistant Commissioner Mark Rowley, sounded a note of profound concern when he called on “every strand of our society” — universities, shop-owners, councils, hospitals and the public at large — to unite in defeating the threat of extremism. In the previous six months, he noted, 36 people had been killed and more than 200 injured in four separate terrorist attacks on British soil. The number of attacks rose to five a few days later when a home-made bomb was left on a London Underground train.

What made Rowley’s concern — and his appeal to his citizens — particularly poignant is that it was expressed not in London but in Herzliya, just north of Tel Aviv, where he was attending the annual conference of the International Institute for Counter-Terrorism.